Conventionally, a converter is constructed of a diode-clamped three-level circuit in many cases. A low-loss device, such as a silicon carbide element, that is under development in recent years is expected to be applied to such a converter to contribute to size reduction of a converter device.
However, although some that can withstand high voltage have started to be available among silicon carbide elements and the like currently provided, those are expensive and need to be tested for reliability. For this reason, under the current situation, use of a silicon carbide element that has low breakdown voltage is practical. When such an element is used, it is necessary that elements be connected in series or be connected in multiple levels in combination with existing silicon elements. When these two options are considered, connecting elements in series involves such disadvantages as an increase in loss, an increase in the number of elements, and balance control. Connecting elements in multiple levels is therefore more advantageous under the current situation.
Given this situation, a multilevel circuit that includes a single-phase three-level converter and a single-phase two-level converter connected in series has been proposed.
In a conventional technique, however, a cooling device needs to be included for cooling power modules included in the single-phase two-level converter and the single-phase three-level converter, respectively, that are included in the multilevel circuit. When this cooling device is exposed to the outside environment so as to perform cooling by use of traveling wind of a vehicle, this cooling device needs to be grounded. In this case, a potential difference occurs between the cooling device and each of the power modules. Therefore, a measure such as provision of an insulation medium between the cooling device and the power module needs to be taken. It can be considered that such a measure not only complicates the structure but also deteriorates the cooling performance because of the insulating medium interposed therebetween.